Behavior-altering parasites and parasitoids

Behavior-altering parasites are parasites capable of causing changes in the behavior of their hosts by directly affecting the hosts' decision-making and behavior control mechanisms. The acquired or modified behaviors assist in the parasite's transmission, and in the case of parasitoids result in the host's death.

Parasites can cause various types of behavioral changes, some extremely complex and "unnatural" to the host. The protozoan Toxoplasma gondii, for example, infects small rodents and causes them to become careless, and attracted to the smell of feline urine, which increases their risk of predation and the parasite's chance of infecting a cat, its definitive host.

Mechanisms of altering the host's behavior include infection of the host's central nervous system and altered neurochemical communication.

Parasites may alter hosts' behaviors in ways that increase their likelihood of transmission (e.g. by the host being ingested by a predator); result in the parasite's release at appropriate sites (e.g. by changes in the host's preferences for habitats); increase parasite survival or increase the host's likelihood of being infected with more parasites.

The protozoan Toxoplasma gondii infects animals from the Felidae family (its definitive host), and its oocysts are shed with the host's feces. When a rodent consumes the fecal matter it gets infected with the parasite (becoming its intermediate host). The rodent subsequently becomes more extroverted and less fearful of felines, increasing its chance of predation and the parasite's chance of completing its lifecycle. There is some evidence that T. gondii, when infecting humans, alters their behavior in similar ways to rodents; it has also been linked to cases of schizophrenia. Other parasites that increase their host's risk of predation include Euhaplorchis californiensis, Dicrocoelium dendriticum, Myrmeconema neotropicum and Diplostomum pseudospathaceum.

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